In the papermaking industry, wood logs are converted into chips, which are subsequently treated in a digester system to separate the cellulose fibers and to remove desired amounts of lignin, etc., which binds the fibers together in the natural state of wood, for the production of paper pulp.
Digestion of wood chips employing an alkaline liquor is a common practice in the industry. In this process, commonly wood chips and an alkaline digesting liquor, sometimes premixed, are introduced to a top inlet zone of a continuous digestion vessel (a digester). In the digestion process, the chips and liquor move generally, but not always, together downward through the digester, the digestion reaching generally optimal completion when the mass reaches the bottom portion of the digester. A typical digester is divided into various zones such as the inlet zone, an upper digestion zone within which, among other things, the chip/liquor mass is heated toward a full cook temperature, a full cook zone within which the mass is subjected to a full cook temperature for a selected period of time, an extraction zone within which digestion spent liquor (black liquor at this point) is withdrawn from the digester, a wash zone in which the mass is washed with process liquids to wash the dissolved solids in the black liquor from the mass, and a withdrawal zone in which the mass of (partially) washed pulp is withdrawn from the digester and passed to further treatment apparatus, such as pulp washers.
Scaling occurs on surfaces of the equipment in an alkaline pulping system and results in loss in productivity and higher operating costs. Severe scaling in a continuous digester system often leads to loss of production of up to several days a year for scale removal by acid cleaning or high-pressure hydro blasting. Currently there are no known cost-effective process modifications to prevent scaling from forming, and many mills rely on the use of a class of expensive chemicals, known as “antiscalants” in the art, as pulping additives to suppress scaling. Even with the antiscalants, costly periodic cleaning of heaters or other digester equipment is often required.
Calcium carbonate has been shown to be a key component of scale formed on surfaces of alkaline pulping equipment such as digester cooking heaters and digester screens. In addition, wood generally is the single largest source of calcium present in cooking liquor. The solubility of calcium salts in alkaline pulping liquor has been found first increases and then decreases with increasing cooking temperature and/or cooking time. When the amount of calcium in the cooking liquor exceeds its solubility, calcium precipitates as calcium carbonate and, along with lignin and other deposits, forms scale on the surface of heater, screens and digester shell wall. Thus, under typical alkaline pulping conditions, the amount of dissolved calcium in the cooking liquor increases as cooking proceeds, goes through a maximum near when the maximum cooking temperature is reached, and decreases rapidly afterward as a result of calcium carbonate precipitation onto equipment surfaces (scaling) and surfaces of chips/fibers.
Scaling tendency of calcium in cooking liquor has been shown to decrease dramatically after the liquor has been heated at or near typical full cooking temperatures. This action is, at times, referred to in the art as calcium deactivation by heat treatment, and has been practiced in some digesters. An exemplary application of this calcium deactivation, as described in European Patent Application EP 0313730 A1, comprises of heating cooking liquor high in calcium at or near full cooking temperature, holding it at this temperature in a vessel for a period of time, typically longer than ten minutes, and returning the heat treated liquor, with “deactivated” calcium, to the digester system. Because scale forms on the surfaces of this “sacrificial” vessel, generally at least two vessels are needed in order to maintain continuous operation of calcium deactivation, with at least one vessel being online and one vessel being cleaned of scales. This technology is probably effective, but requires addition capital and operating costs, and therefore is not widely practiced in the industry.
Cleaning accumulated scale from a digester requires taking the digester offline and removal of the scale, commonly by chemical dissolution of the scale and/or pressure cleaning with a liquid. This cleaning consumes several days of downtime of the digester in addition to the labor required to perform the cleaning, both of which are very costly. As a consequence of such cost, cleaning of digesters is commonly conducted no more frequently than annually. The gradual accumulation of scale within the digester over the period of a year results in ever increasing loss of efficiency as more and more scale develops. It is therefore most desirable that a method be provided for reducing or substantially eliminating the accumulation of scale within a digester.